where the negative term \(-A/r^{6}\)  leads to van der Waals attractive fields and comes from the second-order correction in perturbation theory to the dipole-dipole interaction between two atoms [1]}. The positive
term \(B/r^{12}\)  models the short range electronic repulsion between atoms and has no theoretical justification; it was empirically chosen because it fits reasonably good data coming from experiments with diatomic gases [2]}. An alternative version is given by
\(V_{12-6}(r) = 4\epsilon \left[ \left( \frac{\sigma }{r}\right)^{12}-\left( \frac{\sigma }{r}\right) ^{6}\right], \qquad A =4\epsilon \sigma ^{6} ,\qquad B=4\epsilon \sigma ^{12},\) 
